As is well known in the art, the degree of integration in semiconductor integrated circuits has marked remarkable progresses in recent years. In unison with this tendency, the light source used in the lithography process for the fabrication of semiconductor devices has achieved a substantial reduction of exposure wavelength. The current mainstream lithography uses an ArF excimer laser of 193 nm wavelength. In the drive to achieve a higher degree of integration, the lithography using ArF excimer laser may survive in further developed forms like immersion lithography and double patterning lithography, and a subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising.
The EUV lithography is expected to use a light source of soft x-ray having a wavelength of up to 70 nm, specifically around 13 nm. Since there are no materials which are highly transmissive in this wavelength range, the EUV lithography has to employ a reflecting optical system. While reflection occurs in this system by a reflective multi-layer film of silicon, molybdenum and other elements deposited on a substrate, a fraction (several tens percents) of incident EUV radiation will not be reflected and reach the underlying substrate where it transforms into heat. Since the EUV lithography uses an extremely short wavelength light source as compared with the conventional lithography technology, the lithography precision can be adversely affected even by a slight thermal expansion of each member (e.g., substrate) in the lithographic optical system induced by the heat that has reached there. Accordingly, members like reflecting mirrors, masks, and stages must be made of low expansion materials. Titania-doped quartz glass is a typical low expansion material. The addition of a certain amount of titania makes it possible to minimize the thermal expansion of quartz glass.
It was anticipated in the art that the temperature rise that substrates experience upon receipt of incident EUV radiation is up to 5° C. It was thus believed that EUV lithography members should be reduced in thermal expansion at a room temperature level (about 19 to 25° C.). However, commercially available EUV lithography tools, that is, exposure tools with increased throughputs are expected to elevate the substrate temperature to about 50 to 80° C. Accordingly, it is necessary to correct the temperature at which the substrate undergoes zero expansion and to develop a material which is reduced in thermal expansion over a wider temperature range on the order of −50° C. to 150° C.
One effective means for broadening the temperature range where titania-doped quartz glass has reduced thermal expansion is by doping titania-doped quartz glass with fluorine and lowering its fictive temperature as disclosed in JP-A 2005-104820. The means of doping titania-doped quartz glass with fluorine and lowering its fictive temperature, however, do not always result in titania-doped quartz glass having a wide temperature range for low thermal expansion. The method of doping titania-doped quartz glass with fluorine is generally by previously providing a titania-doped amorphous silica matrix material, exposing the matrix material to an atmosphere containing fluorine, for example, SiF4, heating and vitrifying. However, SiF4 and similar fluorine-containing gases are often expensive and add to the cost of manufacture.